The invention relates to a tunable electrical filter with multiple filter sections which may be used in a cable information system.
Conventional tunable filters or cable traps of the kind described are typically referred to as "tuned notch filters" which are utilized for removing frequency scrambling signals provided within, for example, a TV channel band to eliminate reception of that channel. This is referred to as positive trapping in the art. An exemplary scrambling system for cable television signals is described in U.S. Pat. No. 4,074,311 issued to Tanner et al., incorporated herein by reference. These filters may also be utilized for negative trapping, which involves removing specific frequencies within the TV channel band, such as the video carrier, in order to prevent reception of the channel information.
Such tunable notch filters should be capable of approximately 80 dB of attenuation at the center frequency of the notch, and in all cases should be capable of 60 dB at the scramble signal frequency, the notch being sufficiently narrow to prevent serious degradation of the video information. In order to achieve such levels of notch attenuation, conventional filters have been designed with one or more filter sections which are employed in cascade to achieve the high attenuation required. Problems in these designs, however, arose due to the fact that the tuning of one filter section necessarily affects the electrical performance of the other filter sections because of magnetic coupling between the sections, which in turn increases the time required to tune the overall filter in cascade. One such example of a conventional tunable filter with multiple sections is described in U.S. Pat. No. 4,451,803 issued to Holdsworth et al., incorporated herein by reference.
With reference now to FIG. 1, a conventional multi-section tunable filter 10 of the type described above is shown. The filter 10 includes first 11 and second 12 filter sections, each section respectively including circuit boards 13,14 which are magnetically isolated from one another by isolation shields 15,16, and separated by an isolation area IA. A connecting wire CW electrically connects the circuit boards of each section through the isolation area.
According to the filter illustrated, the first filter section 11 includes a female input terminal 17 and a female connector housing 19 connected to the circuit board 13. A primary coil-post assembly 25 and tuning components 27, e.g., capacitors and secondary coils, are provided on the circuit board 13 for tuning the filter section to a desired frequency notch, the primary coils being tuned by adjustment of tuning set screws 29. The assembled circuit board is enclosed by a female cap 21 which is soldered to the first isolation shield 15 at the open end thereof. The female cap includes an externally threaded port 23 for coupling to the information-carrying cable.
Similarly, the second filter section 12 includes a primary coil-post assembly 26, tuning components 28, and a male output terminal pin 18 provided on the circuit board 14. The circuit board 14 is housed within a male cap 22 with an internally threaded port 24. As with the female cap 21, the male cap includes through holes so that an operator may tune the filter section 12 via the primary coils by adjusting the set screws 30. A moisture barrier seal 20a and outer seal 20b are provided about the output terminal pin 18 in order to protect the filter section from moisture or vapor. It will be appreciated that the assembled filter sections 11 and 12 are inserted and sealed within an outer housing sleeve 31.
The use of filters with multiple filter sections requires that each section be magnetically isolated from one another in order to enhance the ease of tuning of the separate sections and to minimize the effects of tuning of one filter section on the tuning of the other. This magnetic isolation is accomplished by providing the isolation area IA between the filter sections, preferably with the use of spaced-apart isolation shields mounted between the filter sections. The isolation area disrupts magnetic fields which might couple the separate filter sections together, thus minimizing the effects of tuning one of the filter sections on the tuning of the other. Specifically, while the magnetic flux emanating from the sides of each coil within a given filter section does not disrupt the tuning characteristics of coils in the same section due to phase cancellation, the side flux emanations tend to disrupt the overall tuning characteristics of the filter if no shields are used.
Due to specified standards defined within the industry, specifically diameter limitations, conventional filter traps have typically been designed with an elongated housing. This elongation is due in part to achieving optimum distances between the tuning coils associated with each filter section. This separation distance is somewhat limited due to the constraints required for the width of the housing, while there are limited constraints on length. Furthermore, the conventional designs require an adequate isolation area, which may be a space between two shields or a relatively thick single shield, in order that the filter sections are properly isolated from one another.
With the number of channels being offered to subscribers continually increasing, it is advantageous to obtain filters of small physical size. Typically, one filter will be used for each channel to be blocked. As the need for more channels to be filtered increases, the number of filters used increases accordingly. This may create problems for cable television systems using enclosures having limited space for housing the filters and other associated equipment. The weight of the filters is also of concern since the equipments used to interface with the filters are normally capable of supporting minimal stress.
Another drawback to the conventional designs of filter traps in the prior art is the fact that typically the filter sections and their associated inner housings are coupled to one another prior to the tuning of each filter section. Therefore, while conventional designs allow for separate tuning of each filter section, the filter sections remain physically tied to one another. This is quite disadvantageous in situations where one of the filter sections is either inoperable or malfunctioning due to unforeseen circumstances. Unfortunately, since the sections are physically coupled to one another prior to the tuning, the entire assembly of both filter sections is unsalvageable. In those filters which have sections that are capable of being tuned separately and then coupled together, there is a further disadvantage in that the coupling configuration does not prevent access to the tuning screws. These screws are typically accessible from the longitudinal periphery of the filter sections when the outer sleeve is removed.
In addition, when filters of this type are deployed for use, they are typically exposed to an outdoor environment and should be self-protective against atmospheric conditions. When there is a fluctuation in temperature, the components housed within the filter expand and contract. Since the components are constructed from dissimilar materials, the coefficients of thermal expansion for these materials are not unified, thus leading to microscopic openings at the filter interface ports. These openings will allow the penetration of water vapor into the filter, thus altering the pre-set resonant frequency, and rendering the filter ineffective for its application.
It is therefore an object of the present invention to provide an electrical filter having a plurality of filter sections with unique component configurations that allow for the reduction of the overall length of the filter.
It is also an object of the present invention to provide an electrical filter having filter sections which are independently tunable and which are coupled to one another subsequent to the tuning process in a manner which prevents access to the tuning screws of each filter section while the sections are coupled to one another.
It is a further object of the present invention to provide an electrical filter which is adequately sealed so as to prevent water vapor from penetrating the insides of the filter.